Jochen and everybody, hi,
I don't know if it has already been covered on this thread, and I
understand that the following comment is not central to the main
theme. But to group Chernobyl with other reactor failures has one
element that is potentially problematic. Chernobyl was a carbon-
moderated reactor, in contrast to (as far as I know, all other)
modern reactor designs, which are water-moderated in one way or
another. The problem, of course, with carbon moderation is that if
the reactor overheats, the carbon doesn't reduce that, and if it hits
flashpoint, the carbon is inflammable and ultimately explosive. So
you get this massive chemical explosion vaporizing and distributing
your fuel/waste mixture.
In contrast, water-moderated reactors, if they overheat, boil off the
moderator. Since, in either of these reactors, the point of the
moderator is to slow neutrons to an energy that can be captured for
fissioning, when the moderator is lost, the neutrons remain fast, and
mostly escape, which should mostly or entirely shut the reaction.
This isn't as complete a quench as the cadmium absorber rods that can
be used for some kinds of active control, but it should still make
most of the difference that keeps the reaction within what the
containment system was designed for.
There are a lot of details about this that I don't know, so I don't
understand what the modes are that lead to continued reactor heating
even when the moderator should have been removed. This includes not
understanding what went wrong in the research facility in Japan a few
years ago, that led to the bubbling pot of fuel that had to have
holes shot in it from high-powered rifles outside the building to
sufficiently spread the material to go permanently sub-critical.
For me, the most worrisome reactor in the world right now is in St.
Petersburg. it is the same design and age as Chernobyl (I believe),
and is not only a potential disaster for Russia, but for Finland,
Sweden, and northern continental Europe. That would be my candidate
for replacement.
Eric
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